In optical circuits, signals can be transported in the form of light at different frequencies via light conductors, referred to as waveguides. Here, waveguides are coupled, for example, to resonators, in order to filter out certain discrete signals, frequencies or frequency ranges from the waveguides, for instance. In a very general sense, these filters have the task of limiting and/or transforming a wide spectrum of frequencies present at their inlet and supplying them at their outlet. By coupling a second waveguide to the resonator, the frequencies filtered out of the first waveguide can be diverted to the second waveguide. Thus, with the aid of the resonator, signals are removed from the first waveguide in a targeted manner and transmitted into the second waveguide (drop filter). As a result of the coupling of a second waveguide to the resonator, it is also possible to add additional signals from the second waveguide to the signals already present in the first waveguide (add-drop filter). The role of the waveguides is thereby interchangeable.
One important quality criterion of a filter is how effectively a desired frequency range (width of the resonance) can be transmitted exclusively. It is also important that a filter is available which, in the most accurate manner possible, blocks only those frequencies which lie outside the desired range and which allows the frequencies within the desired range to pass virtually unhindered, that is, losslessly.
A frequently used physical principle for implementing filters involves the utilization of a resonant frequency; accordingly, these types of filters are called resonant filters. These filters use, for example, ring resonators or microsphere resonators, which can also be used as sensors.
What are referred to as ring resonant filters or ring resonators are used for the requirement of mass production. The structures of ring resonant filters of this type are thereby produced from silicon wafers (SIO—silicon on insulator) by etching, for example. This etching can occur in a wet-chemical process or using dry-etching methods (reactive ion etching). With suitable etching processes, waveguides and ring resonators can also lie on different planes, or by the etching of multiple individual wafers that are placed on top of one another and, with respect to the resonators and waveguides, precisely positioned relative to one another and permanently connected to one another.
From DE 100 25 307 A1, an optical grid-assisted add/drop filter is known which comprises a structure of a directional coupler filter having at least two waveguides extending in a closely adjacent manner with different refractive indices. Here, the material of the two waveguides is formed from two different material classes with different optical parameters, wherein different properties of the two materials differ such that different effects occur when they are acted on by the same technical means, and wherein the optical parameters may be altered.
From US 2003/0118270 A1, an optical waveguide coupler is known in which a waveguide is directly coupled to a microsphere resonator under resonance conditions or under non-resonance conditions.
Also, from US 2006/0239614 A1, an optical microresonator coupled to waveguides is known which is composed of a plurality of microcylinders coupled to one another, and each microcylinder contains one resonant waveguide.
From US 2005/0013529 A1, a microring resonator and a method for the manufacture of such a resonator are known. A plurality of microring resonators are produced in that precursor resonator structures are brought into a specific position relative to waveguides and transformed into resonators.
Furthermore, according to M. Pöllinger et al.: Optics Express Vol. 18, (2010) 17764, bottle-like microresonators are known which utilize the Kerr effect, in which two conical waveguides are coupled with the bottle-like microresonator.
Additionally, according to S. Böttner et al.: Optics Letters, Vol. 37, (2012) 5136, rolled microstructures are known for the frequency-selective coupling of two layer stacks on one chip.
Disadvantageous according to the prior art is the costly and time-intensive production of the microresonators, in particular for vertically aligned microresonators, as well as their not yet adequately controllable coupling to waveguides.